A process for producing unsaturated aldehydes and then unsaturated acids from olefins is a typical example of catalytic vapor phase oxidation.
To perform the partial oxidation of olefins, a multimetal oxide containing molybdenum and bismuth or vanadium or a mixture thereof is used as a catalyst. Typically, the partial oxidation of olefins may be exemplified by a process for producing (meth)acrolein and then (meth)acrylic acid by oxidizing propylene or isobutylene, a process for producing phthalic anhydride by oxidizing naphthalene or ortho-xylene or a process for producing maleic anhydride by partially oxidizing benzene, butylene or butadiene.
Generally, propylene or isobutylene is subjected to two-step catalytic vapor phase partial oxidation to form (meth)acrylic acid as a final product. More particularly, in the first step, propylene or isobutylene is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrolein as a main product. In the second step, (meth)acrolein obtained from the preceding step is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrylic acid. The catalyst used in the first step is a catalyst comprising an oxide and/or a composite oxide based on Mo—Bi, which oxidizes propylene or isobutylene to form (meth)acrolein as a main product. Additionally, a part of (meth)acrolein is further oxidized on the same catalyst to form (meth)acrylic acid partially. The catalyst used in the second step is a catalyst comprising an oxide and/or a composite oxide based on Mo—V, which oxidizes (meth)acrolein-containing mixed gas produced in the first step, particularly (meth)acrolein, to form (meth)acrylic acid as a main product.
Reactors for carrying out the above process are realized in such a manner that each of the above two steps are implemented in one system or in two different systems (see U.S. Pat. No. 4,256,783).
Generally, (meth)acrylic acid is reacted with alcohol, to form (meth)acrylate, which is used for paints, fiber formula, coating agent of paper. Especially, a high purity of acrylic acid is used as a raw material for a highly hygroscopic resin, and a demand thereof is recently increasing rapidly.
In general, catalytic vapor phase oxidation is implemented as follows. At least one catalyst in the form of granules is packed into reaction tubes, feed gas is supplied to a reactor through the reaction tubes and the feed gas is in contact with the catalyst in the reaction tubes to perform vapor phase oxidation. Reaction heat generated during the reaction is removed by heat transfer with a heat transfer medium, wherein the temperature of the heat transfer medium is maintained at a predetermined temperature. Particularly, the heat transfer medium for heat exchange is provided on the outer surface of the catalytic tubes to perform heat transfer. A reaction product mixture containing a desired product is collected via a duct and then sent to a purification step. Generally, catalytic vapor phase oxidation is a highly exothermic reaction. Therefore, it is very important to control the reaction temperature in a specific range and to downsize hot spots in the reaction zone.
Vapor phase partial oxidation for producing unsaturated acids from unsaturated aldehydes is an exothermic reaction. Therefore, it has a problem in that a hot spot (a point whose temperature is abnormally high) is generated in the reactor. Such hot spots show a relatively high temperature compared to other parts of the reactor. Accordingly, in hot spots, complete oxidation proceeds rather than partial oxidation, thereby increasing by-products such as COx and decreasing the yield of unsaturated acids. Additionally, excessive heat generated in a hot spot causes migration or sublimation of molybdenum that is a main element of the catalyst, resulting in deposition of molybdenum in a catalytic layer and pressure drop in the catalytic layer, degradation of catalytic activity and in shortening of the lifetime of the catalyst. Therefore, yield of unsaturated acid decreases.
Generally, various methods are known in order to control the excessive heat at a hot spot in a catalytic reaction accompanied with heat generation. Such methods include a method for reducing the amount of feed gas to decrease the space velocity and a method of using a reaction tube having a relatively small inner diameter. However, when the space velocity decreases, it is not possible to obtain high productivity in an industrial scale. When the inner diameter of a reaction tube decreases, it is difficult to manufacture the reactor. Moreover, in the latter case, there are disadvantages of economically unfavorable high cost needed for manufacturing the reactor, and increased time and labor needed for packing a catalyst. For these reasons, there has been a continuous need for and research into a method for producing unsaturated aldehydes and/or unsaturated fatty acids with high yield and high productivity by using a catalyst stably for a long time, while avoiding the above problems according to the known methods.
For example, Such methods include a method for packing a catalytic bed by controlling the volume of catalyst in such a manner that the volume gradually decreases from the inlet to the outlet(Japanese Laid-Open Patent No. Hei9-241209); a method for packing a catalytic bed by controlling the volume of catalyst particle in such a manner that the volume gradually decreases from the inlet to the outlet; a method for preparing acrylic acid by multistage-packing with catalysts having different activities; a method of dividing inside of a reaction tube into a plurality of reaction zones and packing each catalyst therein; a method for packing a catalytic bed by controlling the activity of catalyst in such a manner that the activity gradually increases from the inlet to the outlet(Japanese Laid-Open Patent No. 2000-336060); a method for packing a catalytic bed by controlling the amount of catalyst supported on carrier in such a manner that the activity gradually increases from the inlet to the outlet(US No. 2000-336060); a method for packing a catalytic bed wherein a first reaction zone most adjacent to the inlet is packed with the catalyst having higher activity than that of the second reaction zone adjacent thereto and then the catalytic bed from second reaction zone is packed by controlling the activity of catalyst in such a manner that the activity gradually increases from the inlet to the outlet(Japanese Laid-Open Patent No. 2001-112617). However, the above mentioned methods for minimizing degradation of catalyst and suppressing side reactions by decreasing the temperatures of hot spots are not fully effective to solve the above described problems.
Therefore, there is a continuous need for a method for minimizing degradation of catalyst and side reactions caused by extreme heat generation at a hot spot generated during the catalytic reaction.